Grinding ball for ball mills



June 26,195] D. WESTON GRINDING BALL FOR BALL MILLS Filed April 2, 1947RE R OW O 5 T n, W A

Patented June 26, 1951 UNITED STATES PATENT OFFICE 2,558,327 GRINDINGBALL FOR BALL MILLS David Weston, Toronto, Ontario, Canada ApplicationApril 2, 1947, Serial No. 738,7 85

2 Claims.

The invention relates to a new and useful combined crushing and grindingmaterial, and bodies incorporating the material, for use in crushing andgrinding mills of the type, for example, employed in crushing andcomminuting ore, rock and like materials.

Rod mills, ball mills and like apparatus for crushing and grinding rock,ore, and like material are well known and are widely used and involve,in general, a rotatable cylindrical body or drum charged with a mixtureof feed material to be crushed and comminuted and a ball charge. In suchapparatus, balls constituting the ball charge on rotation of the mill,are carried upwardly, by centrifugal force toward the top of the milland are permitted to fall by gravity, on the mixture of charge and ballsin the bottom. Normally, the speed of rotation of the mill is determinedto obtain a maximum impact crushing and attrition grinding effect. Theball charge thus serves two important functions, it breaks the particlesof the charge by impact in falling, and comminutes the charge byattrition grinding of particles engaged between contacting surfaces ofadjacent balls.

Spherical bodies are most advantageous to obtain an efficient crushingor shattering action as they concentrate on a relatively small surfacearea of the material being crushed. A

spherical body, however, has a minimum grinding action in view of thefact that it will only provide for a point contact and consequentlysteps have been taken in the past to increase the grinding effect ofsuch bodies by generally departing from such a shape and forming bodieswith a multiplicity of plane surfaces or enlarged areas, in an endeavourto increase grinding effect. In all such cases, such expedients tend tohinder the free rotatability of the body necessary to achieve efficientgrinding.

The present invention avoids the disadvantages of the prior art byproviding a combined crushing and grinding body which will provide foroptimum crushing effect by employing a spherical body and optimumgrinding effect achieved through an enlarged grinding surface on thespherical body of such character that free rotatability of the body isnot hindered.

It is therefore a main object of this invention to provide a crushingand grinding body in which the configuration is of an essentiallyspherical shape with at least one enlarged substantially cylindricalcontact surface to provide a body which is freely rotatable in the milland yet provide relatively large contact surfaces for engagement withthe particles of the charge for increasing the rate and efiiciency ofthe grinding operation.

The crushing and grinding medium of the present invention comprises ingeneral: a body essentially spherical in form and in which the surfacethereof consists in a combination of at least one substantiallycylindrical grinding surface and at least two substantially sphericalcrushing surfaces separated by said cylindrical grinding surface, thespherical crushing surfaces each forming part of the same sphere and thesaid cylindrical grinding surface forming a cylindrical band around saidbody having a diameter slightly less than the diameter of said sphere.

An understanding of the invention may be had from the followingdescription, reference being made to the accompanying drawings in which:

Figure 1 is an elevation of a grinding ball embodying the improvementsof the present invention;

Figure 2 is an elevation of a grinding ball embodying a modification ofthe invention;

Figure 3 is a section taken along the line 3-3 of Figure 2;

Figures 4 and 5 are elevations of pairs of the grinding ballsillustrated respectively in Figures 1 and 2; and

Figure 6 is a schematic View of the interior of a ball mill employing aball charge in which some of the balls incorporate the improvement ofthe present invention and others are completely spherical to illustratethe extent of the contacting surfaces of adjacent balls.

Like reference characters refer to like parts throughout thespecification and drawings.

The numeral 26 indicates a crushing and grinding ball of the type usedin mills for grinding or crushing and grinding rock, ore and likematerial. The ball is of spherical shape and is formed, as illustratedin Figure 1, as a combination of at least one substantially cylindricalgrinding surface in the form of the substantially cylindrical band 22extending circumferentially around the centre thereof and at least twosubstantially spherical crushing surfaces separated by the cylindricalgrinding surface 22, the spherical crushing surfaces each forming a partof the same sphere, with at least one substantially cylindrical shapedband 22 extending circumferentially around the centre thereof, thecylindrical grinding surface 22 being of a diameter slightly less thanthe diameter of said. sphere and may be formed at the time of mouldingthe 3 ball or may be formed by cutting or grinding after moulding.

In the modification of the invention illustrated in Figure 2, thespherical body 2201. is formed with two circumferential bands 23 and 23awhich extend circumferentially around the centre of the body andintersect at right angles. The periphery of the body between the outsideedges of the bands 2-3 and 23a is also substantially circular in shape.

In each instance, the enlarged contact surface 22 or surfaces 23-23apresent a contact surface of relatively large area, having regard to thecircumferential area of the ball, without decreasing appreciably thefree rotatability of the ball. For example, in a ball of about 2 indiameter, the width of each band forming the enlarged contact surface isabout one-half inch.

In the use of balls having the configuration described and illustratedherein, the balls are rolled around the bottom of the mill at differentrates of speed and at different angles of direction as the mill isrotated and the substantially spherical shape of the balls does notofier any appreciable resistance to their free rotation. When thecircular portions of two balls come in contact there is the usual pinpoint contact between the adjacent bodies rotating'at different rates ofspeed and a grinding action is effected on the'particies of the chargetherebetween. However, when two enlarged contact surfaces of adjacentballs rotating at different rates of speed come together, the area ofthe contact is large as compared with the pin point contact of thespherical portions with longer time of contact to increase greatly therate and efficiency of the comminuting operation.

In use, the combined crushing and grinding balls of the presentinvention have many important advantages over the balls hitherto usedfor the same purpose.

Reduction by impact on the material results from one body falling orbeing thrown against another with sufficient force so that the lesscoherent body is shattered, breaks, or weakened to the extent that itbreaks or shatters, under succeeding shocks. The design of the impactbody is, therefore, an important factor in the efiiciency of thecrushing or breaking operation in that the greater the concentration ofenergy delivered to the feed material per unit of surface area, thegreater is the shattering, breaking, or weakening effect on the feedmaterial.

Balls of the present invention are designed to provide a substantiallyspherical shape so that in falling, the force of the fall isconcentrated on a relatively small surface area of the material on whichit drops or is thrown.

Reduction of the material by attrition results from two or more of thegrinding bodies coming together with contacting surfaces and mov ing atdifferent speeds or in different angles of direction to reduce the feedmaterial caught between the contacting surfaces of the bodies. The areaof the contacting surfaces of adjacent balls is an important factor inthe efficiency and rate of reduction by attrition. In mills employingthe spherical balls of the prior art, the bodies come together with, ineffect, a pin point contact, that is, the area of contact, or thefriction area is relatively small and only a small amount of material isengaged between the contacting surfaces. By employing balls havingenlarged contact surfaces on the spheres, a much larger area of contactis provided to increase the area of the contacting surfaces, or thefriction area, and so increase the amount of material which may beengaged between them, and the length of time surfaces are in engagementone with the other. a

The balls of the present invention are ideally adapted for designing tomeet the requirements of the feed material with which they are to beused, for example, some types of material, or the same material in itsvarious stages of reduction, will require a greater percentage of impactand others a greater percentage of attrition. In each instance, theballs are designed to have a maximum combined impact and attritionefiect, but the enlarged contact surfaces may be modified to increasethe impact effect, or exaggerated to increase the attrition effectwithout materially decreasing itsattrition effect in the first instanceor the impact effect in the second instance. Also, by retaining thegeneral spherical shape of the balls and using.

tungsten carbide with its extreme hardness and resistance to wear, theballs retain their original shape over long periods of use withnegligible contamination of the final product.

What I claim as new and and desire to protect by Letters Patent of theUnited States is:

1. A crushing and grinding body for use in ball mills and the like, saidbody being essentially spherical in form, and the surface thereofconsisting of a combination of at least one substan- DAVID WESTON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,431,475 MacDonald Oct. 10, 19221,746,236 Barton Feb. 11, 1930 1,823,351 Clements Sept. 15, 19311,864,542 Holzapfel June 28, 1932 2,253,969 Dawihl Aug. 26, 1941 FOREIGNPATENTS Number Country Date 246,653 Italy Mar. 4, 1926 261,664 GreatBritain Nov. 25, 1926 274,786 Great Britain July 28, 1927 569,122

Great Britain May 4, 1945

